Flue gas recirculation (FGR) technology itself is not new. However, the present system is new, and it serves to reduce fuel NO.sub.x emissions far below those found on any commercial boiler.
Existing flue gas recirculation technologies recycle a portion of the relatively cool, low oxygen combustion products from the stack back into the burner. External piping is used to transport the flue gases and mix it with combustion air prior to burning, or, alternatively, to deliver it directly to the combustion zone. This recirculated flue gas acts as a diluent to lower the overall oxygen concentration, to lower the flame temperature, and to lower the residence time at peak temperature. These effects result in large reductions in thermal No.sub.x emissions, but a negligible reduction in fuel No.sub.x emissions. Therefore, flue gas recirculation achieves better reductions with gaseous fuels than with liquid fuels. Because of the greater potential for flame instability and emissions of unburned combustibles with distillate oil firing, recirculation rates are usually limited to approximately 10 to 15%; and, because of this lower recirculation rate and the presence of fuel No.sub.x, oil-fired reductions are typically limited to only 20%.
Normally, one of two designs has been used to achieve FGR. In one, a separate powered fan was used to force the flue gases from the boiler stack into the air plenum or combustion chamber. In the other, the external FGR duct is so placed that the combustion air fan draws both combustion air and flue gases.
In the present invention, we improve the stability and performance limits of the burner by introducing the combustion air and the flue gas tangentially at high velocity into the burner blast tube. The two streams mix thoroughly and are then passed through a diffuser assembly where the air and FGR mixture is directed across natural gas nozzles located peripherally downstream of the diffuser. The combustion process takes place as a high velocity swirling flow down the length of the Morison tube (boiler furnace).